Grating design for use in a seismic sensing system

ABSTRACT

Seismic sensor systems and sensor station topologies, as well as corresponding cable and sensor station components, manufacturing and deployment techniques are provided. For some embodiments, networks of optical ocean bottom seismic (OBS) stations are provided, in which sensor stations are efficiently deployed in a modular fashion as series of array cable modules deployed along a multi-fiber cable.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/381,880filed May 5, 2006, now U.S. Pat. No. 7,366,055 which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention generally relate to seismic sensing and, inparticular, to components and techniques for deploying and interrogatingarrays of seismic sensors, such as in ocean bottom seismic sensing (OBS)applications.

2. Description of the Related Art

Marine seismic exploration surveys for the exploration and monitoring ofhydrocarbon producing zones and reservoirs utilize seismic cables havingsensor arrays, i.e., a plurality of sensor stations interconnected bysections of cable. The cable arrays may include a large number of sensorstations (e.g., several hundreds or thousands) and may be buried in apredetermined pattern on the ocean floor. Optical sensors may beparticularly well suited for ocean bottom seismic (OBS) applications,due to their robust nature, lack of sensitive electronics, and potentialfor light weight sensors and cable assemblies that are relativelyinexpensive to install. An optical sensor station may include opticalhydrophones, accelerometers along multiple axes, and/or geophones.

The individual sensors in a station, such as accelerometers oriented inorthogonal X, Y, and Z axes, may be interferometers. In such systems, alight source generates interrogating light pulse pairs (spaced apart inaccordance with a length of fiber between reflectors in eachinterferometric sensor), resulting in interfering signals reflected backto the surface. These interfering signals may be analyzed by surfaceelectronics, and recorded and interpreted into seismic data.

As the total number of sensors in the arrays increases in high channelcount (HCC) applications, it becomes a challenge to interrogate eachsensor using a manageable number of optical fibers run to and fromsurface instrumentation. While multiplexing techniques, such aswavelength division multiplexing (WDM) and time division multiplexing(TDM) are well known, there are typically limits to each. On the onehand, there is a practical limit as to how many sensors may beinterrogated by a single fiber, due to a limited number of wavelengthsand limitations on total transmitted power per fiber set by opticalnonlinear interactions. On the other hand, TDM of multipleinterferometric sensors using reflectors of a common wavelength aresubject to unwanted reflections between sensor elements (causingcross-talk).

In some cases, in order to generate sufficient optical power tointerrogate a high number of sensors in an OBS array, relativelyexpensive components, such as remotely pumped sources and opticalamplifiers may be used. Unfortunately, such remotely deployed componentsare relatively expensive and typically require special pressure sealedhousings to be operated at the high pressures seen at the ocean bottom.Replacing failing components remotely located subsea is an expensive andtime-consuming process.

Packaging and deployment of OBS sensor arrays also create challenges inorder to achieve efficient coupling of the seismic signals to therespective sensors. Station packaging should ensure sufficientprotection of the sensors during installation and operation, and shouldalso withstand hydrostatic pressures typical at the ocean bottom (e.g.,50-200 bar). The packaging and station design should ensure highreliability over a relatively long expected lifetime and efficientassembly procedures in order to reduce overall manufacturing costs.

Cutting and splicing data transmission cables/fibers within the cablearray at each of the sensor stations increases time and cost whiledecreasing reliability. Design of the sensor station and/or cable arraycan affect how many splices are required at each station. Accordingly,any designs or techniques that reduce the number of splices of the datatransmission cables/fibers at each station decreases assembly time andcost while increasing reliability of the cable array.

Therefore, there exists a need for an inexpensive and improved OBSsensor system with a large number of sensors, as well as correspondingcable and sensor station components, manufacturing and deploymenttechniques.

SUMMARY OF THE INVENTION

Embodiments of the invention generally relate to seismic signalprocessing methods, apparatus and systems.

One embodiment provides a seismic sensing system generally including oneor more series of seismic sensor stations and instrumentation. Eachstation houses a plurality of optical sensors sharing a commonwavelength, the common wavelength being different for each station in asame series. The instrumentation is coupled to the series of seismicsensor stations and configured to interrogate sensors housed in eachstation using time division multiplexing (TDM) and to interrogatesensors housed in different sensor stations in each series usingwavelength division multiplexing (WDM).

Another embodiment provides an array of seismic sensor stationsgenerally including at least one array connection module and a pluralityof array cables extending from the array connection module. Each arraycable includes a series of array cable modules and a multi-fiber leadcable, each array cable module including a series of seismic sensorstations, each station housing a plurality of optical sensors sharing acommon wavelength, the common wavelength being different for eachstation in a same series. The array also includes, for each array cablemodule, a module connection node to couple a different one or more offibers of the lead cable to a sensor fiber used to interrogate a seriesof corresponding seismic sensor stations.

Another embodiment provides an array cable module generally including acable section extending a length of the array cable module and havingplurality of optical fibers and a plurality of seismic sensor stations.Each station houses a plurality of serially connected interferometricsensors sharing a common wavelength, wherein the common wavelength ofeach station is different. The system also includes at least one sensorfiber for interrogating the sensors in each of the stations and a moduleconnection node configured to optically couple at least one of theplurality of fibers of the cable section to the sensor fiber while aremaining one or more of the plurality of fibers of the cable sectionbypass the sensor stations without optical coupling.

Another embodiment provides a method of gathering seismic data from aseries of seismic sensor stations, each station housing a plurality ofoptical sensors sharing a common wavelength, the common wavelength beingdifferent for each station in a same series. The method generallyincludes interrogating, via a common optical path, sensors housed ineach station using time division multiplexing (TDM) and interrogating,via the common optical path, sensors housed in different sensor stationsin each series using wavelength division multiplexing (WDM).

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIGS. 1A and 1B illustrate exemplary ocean bottom seismic (OBS) sensingsystem topologies in accordance with embodiments of the presentinvention.

FIG. 2 illustrates an exemplary sensor array cable module configuration,in accordance with one embodiment of the present invention.

FIG. 3 illustrates a schematic view of an exemplary sensor station, inaccordance with one embodiment of the present invention.

FIG. 4 illustrates a basic configuration of sensors within a sensorstation, in accordance with one embodiment of the present invention.

FIGS. 5A and 5B illustrate a schematic view of instrumentation of thesystem of FIG. 1, in accordance with one embodiment of the presentinvention.

FIG. 6A illustrates an exemplary grating profile, showing threeneighboring wavelength channels, suitable for gratings of a seismicsensor station, in accordance with one embodiment of the presentinvention.

FIG. 6B illustrates an exemplary arrangement of gratings andcorresponding reflectivities within a sensor station, in accordance withone embodiment of the present invention.

FIGS. 7A and 7B illustrate exemplary module connection nodes thatutilize band wavelength division multiplexing (B-WDM), in accordancewith embodiments of the present invention.

FIG. 8 illustrates exemplary groupings of wavelengths within multiplepulse pair time slots, in accordance with embodiments of the presentinvention.

FIG. 9 illustrates exemplary reflected pulses from an array ofreflectors separating sensors in a station cross-talk.

FIGS. 10A-10B illustrate an exemplary OBS sensing system topologyallowing interrogation from two ends, in accordance with one embodimentof the present invention.

FIGS. 11A and 11B illustrates another exemplary OBS sensing systemtopology allowing interrogation from two ends, in accordance with oneembodiment of the present invention.

FIGS. 12A and 12B illustrate an exemplary sensor array cable moduleconfiguration being interrogated in first and second directions,respectively, in accordance with one embodiment of the presentinvention.

FIGS. 13A and 13B illustrate another exemplary sensor array cable moduleconfiguration being interrogated in first and second directions,respectively, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION

Embodiments of the invention generally relate to seismic sensor systemsand sensor station topologies, as well as corresponding cable and sensorstation components, manufacturing and deployment techniques. For someembodiments, networks of optical ocean bottom seismic (OBS) stations areprovided, in which sensor stations are efficiently deployed in a modularfashion as series of array cable modules deployed along a multi-fibercable.

Interferometric sensors within each sensor station may share a commonwavelength and be interrogated in a time division multiplexed (TDM)manner. Each sensor station, however, may utilize a different wavelengthfor its sensors, allowing multiple stations in series within an arraycable module to be interrogated on a common “sensor” fiber utilizingwavelength division multiplexing (WDM).

For some embodiments, within each array cable module, only the sensorfiber is connected to the sensors at each station, while a multi-fibertube “bypasses” each sensor station with no connections and, hence, nocutting or splicing. At a transition point between array cable modules,a module connection node may be used to connect a different fiber fromthe multi-fiber tube to the sensor fiber used to interrogate the nextseries of sensor stations. In such embodiments, only the sensor fiberneeds to be spliced at each sensor station, in order to connect to thesensors for that station.

Ocean bottom seismic (OBS) sensing systems are described below as aparticular, but not limiting, example of an application in whichembodiments of the present invention may be used to advantage. However,those skilled in the art will recognize that the concepts describedherein may be used to similar advantage in a wide variety of otherapplications in which a large number of optical sensors areinterrogated.

Other examples of where similar sensor arrangements that may becontained in a common housing and interrogated via the methods describedherein (such as a tubular element or mandrel) include flow metersutilizing arrays of linearly apart sensors. Such flow meters aredescribed in detail in U.S. Pat. No. 6,785,004, entitled “METHOD ANDAPPARATUS FOR INTERROGATING FIBER OPTIC SENSORS,” commonly owned withthe present application, herein incorporated by reference in itsentirety.

Further, while embodiments of the present invention will be describedwith reference to optical fibers, those skilled in the art willrecognize that any type of suitable optical waveguide may be used aswell. Further, while embodiments of the present invention will bedescribed with reference to sensor elements utilizing inline reflectiveelements such as FBGs to create interferometers, those skilled in theart will recognize that concepts described herein and recited in theclaims may, in some cases, also be applied to interferometers utilizingtransmissive elements (with analogies drawn between transmissiveproperties and reflective properties) and, more generally, to a widevariety of optical sensors.

An Exemplary OBS Sensor System

FIG. 1A illustrates an exemplary OBS system 100 _(A) in accordance withone embodiment of the present invention. The system 100 _(A) includes aninstrumentation unit 110 configured to interrogate an array of sensorstations 142, which may be deployed along a plurality of array cables1060 extending from an array connection module 130A. As illustrated,each array cable 1060 may include a series of array cable modules 140,with each array cable module including a module connection node 144 anda series of sensor stations 142.

For some embodiments, the instrumentation unit 110 may be located on thesea surface (“topside”), for example, on a boat or platform. For otherembodiments, the instrumentation unit 110 may be located underwater, forexample, within a water-tight chamber in the sea (e.g., on the seafloor). In such cases, either optical or electrical cables may be usedto pass processed data from the instrumentation unit to a platform,ship, or to an on shore data recording or processing center.

A lead cable 120 may connect the instrumentation 110 and the arrayconnection module 130A. The lead cable 120 may be a proprietary orstandard cable suitable for sub-sea deployment, of varying lengthdepending on the particular application, for example, ranging from 1-50km. The lead cable 120 may include one or more fibers to carryinterrogating light pulses to the sensor stations and to carry reflectedlight pulses from the sensor.

The total number of fibers in the lead cable 120 may depend, among otherthings, on the total number of array cable modules to be interrogated.As will be described in greater detail below, for some embodiments, thelead cable 120 may include at least two fibers for each array cablemodule, including one for carrying interrogating light pulses and aseparate one for carrying return (e.g., reflected) light pulses. In suchcases, array connection modules may include connections to couple twofibers from the lead cable 120 to each array cable module in an arraycable. Further, the type of fibers contained therein may be selectedbased on a number of factors, such as non-linearity, polarizationproperties and overall loss.

For some embodiments, the lead cable 120 may be deployed while separatedfrom the array cable modules 140 (e.g., on the ocean bottom) and laterbe connected to the array cable modules. In such embodiments, the arrayconnection module 130A may be referred to as a “wet connection” nodebecause the connection is made sub-sea. The array connection module 130Amay have a pressure sealed housing containing a fiber distributionnetwork, with a different group of fibers 141 routed to interrogatesensor stations 142 along correspondingly different cable arrays 1060.The array connection module 130A may also comprise optical connectors,for example wet-mate connectors. As will be described in greater detailbelow, a module connection node 144 of each array cable module 140 mayconnect a different fiber from a fiber group 141 to a sensor fiber 146used to interrogate all sensor stations 142 within a single array cablemodule 140.

As illustrated in FIG. 1B, for some embodiments, multiple arrayconnection modules 130B may be utilized, with one or more array cables1060 (and corresponding series of array cable modules 140) extendingfrom each. Similar or the same type of array cables 1060 and/or arraycable modules 140 may be utilized in either topography shown in FIG. 1Aor FIG. 1B. In practice, the particular choice of network topology(e.g., between that shown in FIG. 1A, FIG. 1B, or some other type oftopology) will typically depend on the oil field/reservoir topology tobe mapped and existing seabed infrastructure.

For some embodiments, sensor stations 142 within a series of array cablemodules 140 may be interrogated utilizing a combination of bothwavelength division multiplexing (WDM) and time division multiplexing(TDM). As an example, various sensors within a station 142 (e.g., x, y,z accelerometers, a reference interferometer and a hydrophone as shownin FIG. 3) may be interferometric sensors with reflective elements(e.g., gratings) that share a common wavelength and, thus, may beinterrogated via TDM.

However, as illustrated in FIG. 2, each of the (N) different stations142 within an array cable module 140 (interrogated with a common sensorfiber 146) may use a different wavelength (e.g., λ1-λN) for its sensors.Thus, multiple stations 142 within an array cable module 140 may beinterrogated, via WDM, using a common sensor fiber 146. The number ofstations (N) may be limited by a variety of parameters, such as thespectral bandwidth available, the amount of loss through each station,and total fiber length with signals propagating in both directions(Rayleigh scattering). In any case, a total length of severalkilometers, with up to 10-100 stations per array cable module areachievable, for example, with N=20 in one embodiment.

Different sensor stations (i.e. interrogated on different wavelengthchannels) will experience a different loss depending on their positionwithin the array cable. Thus, for some embodiments, wavelength channelordering in the sensor array may be controlled in an effort to reducecross-talk. The order of the wavelengths within an array cable modulecan be selected to be any possible order of N different wavelengths.

Loss contributed from splices may be controlled by reducing the overallnumber of splices required in the system. For one embodiment, the onlysplices required at each station 142 may be to couple a single sensorfiber 146 (shared with other stations 142 in the same array cable module140) to the sensors of that station. A remaining set of fibers may“bypass” the sensors in the station in an uncut multi-fiber tube 148(e.g., a fiber in metal tube or FIMT).

As illustrated in FIG. 2, at a transition between array cable modules140, a module connection node 144 may be used to couple a differentfiber from the multi-fiber tube 148 to the sensor fiber 146 of asubsequent array cable module 140. The module connection node 144 mayutilize any suitable components for such a transition, such as anoptical circulator 149, optical coupler, wavelength multiplexer and thelike. Further, as will be described in greater detail below, withreference to FIGS. 12 and 13, the exact components may depend on theparticular array topology utilized for a given embodiment.

In any case, a different pair of fibers of the multi-fiber tube 148(where one fiber in the pair is used for down lead and one for up lead)may be used to interrogate the N sensor stations 142 (e.g., withwavelengths λ1-λN) of each different array cable module. Thus, N sensorstations may be interrogated per pair of fibers in the multi-fiber tube148, with cutting into the multi-fiber tube 148 to couple a new pair offibers to the sensor fiber 146 of a subsequent array cable module (viacirculator(s), coupler(s), WDM(s) or similar components) occurring onlyat the module connection nodes 144. This reduced number of splicessimplifies overall array design and may significantly reducemanufacturing costs. This also reduces transmission losses to and fromthe array cable modules, reducing problems with optical nonlinearity andthe need for expensive optical power amplifiers. The module connectionnodes 144 may be separate components, or for some embodiments, may beintegrated within a seismic station 142, providing a compact andefficient design. Particular designs, as well as methods for suchsplicing and corresponding management of a sensor fiber and multi-fibertube is described in detail in the commonly-owned U.S. patentapplication Ser. No. 11/313,275, filed Dec. 20, 2005 entitled “OCEANBOTTOM SEISMIC STATION.”

An Exemplary Seismic Sensor Arrangement

FIG. 3 illustrates a schematic view of an exemplary arrangement ofseismic sensors within a seismic sensor station housing 200, inaccordance with one embodiment of the present invention. As illustratedin FIG. 3, a section of sensor fiber 146 leading to or from a sensorstation 144 may be spliced onto a sensor fiber section that passesthrough the sensor station housing 200. As illustrated, the housing 200may include a variety of different sensors, to which the sensor fiber146 may be connected with only two splices 201. At the last station (λN)in an array cable module 140, the sensor fiber 146 may be connected to aterminating connection 203.

As illustrated in FIG. 2, additional fibers may pass through the station144 without connection to the sensors, thus avoiding cutting andsplicing for those fibers. Each of these fibers may be later connectedto a sensor fiber 146 of a subsequent array cable module 140 in theseries and used to interrogate sensors therein. As illustrated by thesmall dashed lines, after a fiber has been connected to an array cablemodule 140, that fiber may be subsequently left unconnected.

In the illustrated arrangement, the housing 200 contains a referenceinterferometer 210, orthogonal X, Y, and Z accelerometers 220 (220X,22Y, and 220Z, respectively), and a hydrophone 230. As described in theabove-referenced application, the accelerometers 220 may be arranged insome type of liquid filled compartment of the housing 200 for dampeningof mechanical resonances caused by mechanical disturbances and pressurefluctuations. The housing may also include a mechanism for transferringpressure variations between the surrounding environment and an inside ofa second compartment containing the optical fiber coil of the hydrophone230. The reference interferometer can be used to compensate forinterrogating laser frequency fluctuations or phase perturbations in acompensating interferometer 528 (described in greater detail below) orin the lead cable as described in the commonly-owned U.S. patentapplication Ser. No. 10/693,619, filed Oct. 24, 2003 entitled “DownholeOptical Sensor System with Reference.”

Each of the sensors 210, 220, and 230, may be formed by a length offiber (e.g., a coil) separating a pair of gratings 202 formed therein.For some embodiments, the gratings 202 may be fiber Bragg gratings(FBGs). Further, as will be described in greater detail with referenceto FIGS. 6A and 6B, the optical properties of the gratings, includingthe features of the reflective spectrum, may be controlled to reducecross-talk between sensors within the same station, as well as sensorsfrom other stations.

While each sensor may be formed by two gratings, gratings may be sharedbetween sensors, such that only M+1 gratings are required for Minterferometric sensors. For example, in the illustrated arrangement,six gratings with overlapping channel (reflection) bands are used toform the five sensors shown.

As illustrated in FIG. 4, the reference interferometer 210 may be formedby a coil separating gratings 202 ₁ and 202 ₂. The x, y, and zaccelerometers 220 may be formed by coils separating gratings 202 ₂ and202 ₃, 202 ₃ and 202 ₄, and 202 ₄ and 202 ₅, respectively, while thehydrophone 230 may be formed by a coil separating gratings 202 ₅ and 202₆. For some embodiments, the gratings may be formed in the fibersection, with appropriate spacing prior to wrapping the coils, resultingin spacing (optical path length), L, after wrapping. Forming thegratings in this manner may eliminate the need for splices betweensensors, reducing loss, manufacturing time and, thus, overall cost.

Any change in the optical path lengths between the fiber Bragg gratings,as will typically result from external influences on the accelerometeror hydrophone fiber coils, will alter the resulting superposed reflectedsignal from one seismic stations. U.S. Patent Publication No.2005/0097955, describes examples of interferometric accelerometers fordetermining acceleration and methods of fabricating such accelerometers.The accelerometers are based on a rigid frame, a mass movably suspendedon the rigid frame and a sensing coil partially wrapped around surfacesof first and second elements to detect movement of the mass in responseto an acceleration based on a change in length of the sensing coil.

A general problem of arranging a hydrophone together with x, y, and zaccelerometers (or geophones) in a four-component (4-C) seismic sensingstation is the cross sensitivity between the hydrophone and theaccelerometers. While it is generally desirable to optimize the exposureof the hydrophone to pressure variations it is generally undesirable tolet pressure variations influence the geophones/accelerometers. Theinfluence of the pressure signal on the geophones/accelerometers createsan undesirable cross sensitivity. For some embodiments, the sensorstation and housing may be designed to reduce such influence, and may bedesigned in accordance with one of the seismic sensor station housingsdescribed in detail in the commonly-owned U.S. patent application Ser.No. 11/381,922, filed May 5, 2006, entitled “Seabed Seismic StationPackaging” herewith.

Referring back to FIG. 3, in order to interrogate the sensors, anoptical “double pulse” signal 310 of a wavelength (λ1 in the illustratedexample) within the channel (reflection) bandwidth of the gratings 202₁-202 ₆ is launched into the optical fiber section inside the housing200. The time delay between the two pulses is chosen to match theoptical propagation (round trip) delay between each pair of consecutivegratings 202. Thus, in the optical signal reflected from the gratings, areflection of the second pulse from the first fiber grating willsuperimpose on the reflection of the first pulse from the second fiberBragg grating.

For the illustrated arrangement of sensors, utilizing a total of sixgratings 202, a total of five interfering (superimposed reflected)pulses 320 will be produced containing the sensor signals followed bytrailing pulses 322. In some cases, sensor cross-talk may be caused bymultiple reflections (i.e., reflected pulses that are reflected againand interfere with another pulse). As will be described in greaterdetail below with reference to FIG. 9, measures may be taken to reducethe impact of such cross-talk, for example, by applying an inversescattering algorithm, such as layer-peeling, within the instrumentationunit.

As previously described, several seismic stations may be interrogatedvia a common sensor fiber 146 using wavelength multiplexing, by choosingdifferent wavelengths for the gratings of each seismic sensor station.Illustratively, the gratings of the shown seismic sensor station is setat a first optical wavelength λ1, while the interrogating optical signalmay comprise light at other wavelengths λ2, λ3, λ4, . . . λN, intendedfor other seismic sensor stations and will pass virtually un-reflectedthrough the shown seismic station.

The embodiments described above utilize TDM within each station and WDMbetween stations. As an alternative, some embodiments may utilize amultiplexing configuration employing WDM within each station and TDMbetween each station. General concepts of such a multiplexing scheme aredescribed in U.S. Pat. No. 5,987,197, herein incorporated by reference.When compared to the multiplexing scheme shown in the figures anddescribed above, a scheme utilizing WDM within a sensor station willrequire additional gratings (and possibly additional splices), asgratings with different wavelengths will not be shared between seriallyconnected interferometers. Further, for some embodiments, within thesame station, a plurality of sensors sharing a common wavelength may beinterrogated via TDM, while one or more sensors having differentwavelengths may be interrogated via WDM.

Exemplary Instrumentation

FIG. 5A illustrates a schematic view of the instrumentation unit 110 ofthe system of FIG. 1, in accordance with one embodiment of the presentinvention. In general, the collective components in the instrumentationis designed to generate interrogating light pulse pairs with wavelengths(λ1-λN) corresponding to the sensor stations, as described above, andprocess the resulting interfering reflected pulses to extract seismicdata therefrom. While the instrumentation unit 110 shown includescomponents for performing both of these functions, for some embodiments,separate components performing the pulse generation and signalprocessing functions may be provided in separate units.

As illustrated, the instrumentation may include a light source 510capable of producing light signals with multiple wavelengths (λ1-λN).The light source 510 may include any suitable components, such asmultiple fiber lasers, to generate suitable light signals. Suitablelight signals may include, for example, continuous wave light signalswith low intensity and frequency fluctuations, unless coherencemodulation may be applied directly to each wavelength inside the lightsource (as will be discussed in greater detail below). For someembodiments, the light source 510 may be configured with a highbirefringence polarization maintaining output fiber with thepolarization of the fibers output light aligned with one of thebirefringence axes.

As illustrated, separate light signals at different wavelengths may beoutput to one or more of modulator channels 520. If multiple modulatorchannels are used, different wavelengths may input to each modulatorchannel. As illustrated in FIG. 5B, multiple wavelengths input to onemodulator channel may be combined by a wavelength division multiplexing(WDM) unit 522 to combine the signals of multiple wavelengths onto acommon fiber. Each modulator channel 520 may include any suitablecomponents to shape the amplitude, coherence properties, phase andpolarization state of the light signals generated by the source 510. Forsome embodiments, a modulator channel 520 may include an intensitymodulator unit 523 and a coherence modulation unit 524 to shape opticalpulses and control their coherence properties, amplifier(s) 526 tocompensate for losses in the modulators. A modulator channel may alsoinclude a compensating interferometer (CIF) 528 to split single pulsesinto double pulses, phase modulator(s) 532 to control the phase of thepulses and polarization modulator(s) 534 to control the polarizationstate of the output light. The exact type of modulators used may depend,for example, on the output of the light source. For example, assumingthe light source 510 is configured to produce polarization maintainedlight signals, particular modulators, such as Lithium Niobatemodulators, may be used for one or more of items 523, 524, 532 and 534.

As described in the commonly-owned U.S. patent application Ser. No.10/961,326, entitled “Active Coherence Reduction for InterferometerInterrogation,” herein incorporated by reference in its entirety, thecomplex field amplitude of the signal interrogating an opticalinterferometer may be modulated (coherence modulation) in such a waythat the temporal coherence is reduced, thus reducing the sensitivity tounwanted reflections with time delays that are different from the sensorreflector. For some embodiments, the optical field phasor of the lightsource 510 may be modulated in a controlled manner to produce abroadened optical source power spectrum. This may be achieved throughsome direct source modulation, for instance through modulation of lasercavity parameters changing the laser frequency or phase or throughmodulation of laser pump signals. It can also be achieved throughmodulation of the light inside the coherence modulation unit 523 thatmay be included in the modulation channel(s).

A compensating interferometer (CIF) 528, having a delay differencesimilar to the delay difference of sensors in the sensor stations 142may be arranged in a serially coupled manner at the output of the signalconditioning logic 520 to produce pulse pairs suitable for producinginterfering reflected pulses from the gratings in the sensor stations142.

The output from the compensating interferometer 528 may be sent toadditional modulators, which may include suitable components, such asphase modulator 532 and polarization modulator 534 to modulate the phasedifference between pulses in each pulse pair that will result insubcarrier modulation of the interference signals reflected from thesensors allowing for sensor phase demodulation without ambiguity, and toperform polarization conditioning for polarization insensitive sensorinterrogation, for example, in accordance with commonly owned U.S.patent application Ser. Nos. 10/649,590 and 11/056,970, entitled “METHODAND APPARATUS FOR PRODUCING DEPOLARIZED LIGHT,” and “METHOD ANDAPPARATUS FOR PROVIDING POLARIZATION INSENSITIVE SIGNAL PROCESSING FORINTERFEROMETRIC SENSORS.” In addition, the common phase or frequency ofinterrogating pulse pairs can be modulated to reduce cross-talk andnoise caused by unwanted reflections in the system, according to thecommonly owned U.S. patent application Ser. No. 11/056,970, entitled“METHOD AND APPARATUS FOR SUPPRESSION OF CROSS-TALK AND NOISE INTIME-DIVISION MULTIPLEXED INTERFEROMETRIC SENSOR SYSTEMS,” all of whichare herein incorporated by reference in their entirety.

As an option, for some embodiments, the instrumentation may includemeans for spreading the different wavelength channels out in time (e.g.,distributing them in different pulse pairs). Spreading the wavelengthsout in this manner may reduce peak optical power levels, and hencereduce non-linear effects, such as stimulated Raman scattering (SRS),four wave mixing (FWM), self-phase modulation and cross-phase modulation(SPM/XPM), in the fibers, which can degrade the system performance

The wavelengths can be spread out in time by grouping the wavelengths,e.g. with λ1-λ4 in group 1, λ5-λ8 in group 2, etc. Then each group canallocate a different time slot. As an example, a TDM rate of 2000 ns and300 ns duration of each pulse-pair allows for 6 time slots, asillustrated in FIG. 8.

Allocation of different wavelengths to different time slots can beachieved by transmitting different groups of wavelengths throughdifferent modulator channels 520, as suggested in FIG. 5, and activatingthe modulators in different channels to generate pulses at differenttimes. In some cases, this may be beneficial by allowing each modulatorto be optimized for a limited wavelength range. Alternatively, spreadingof wavelengths in time can be achieved by having different opticaldelays (fiber coils with different lengths) for different groups ofwavelengths.

In any case, referring back to FIG. 5A, the modulated signals fromgroups of wavelengths in different wavelength bands may then be fed intoa wavelength division multiplexing (WDM) unit 538, to combine allwavelengths into one fiber. The output from 538 may be passed through abranching module 540 that contain a splitter 544 that divides themulti-pulse multi-wavelength signal into multiple fibers. Severalbranching modules 540 may be cascaded in a tree topology to split theinterrogation signal into a required number of down lead fibers 122 thatmay be combined into one or more down lead cables 120. Note that a downlead cable may in many cases also serve as an uplead cable containinguplead fibers in addition to downlead fibers. Some branching units mayinclude a broadband optical power amplifier 542 to compensate for thesplitting loss and generate required output powers. It may also bepossible to amplify signals and maybe also compensate for splitting lossat a later point (e.g., with locally or remotely pumped subseaamplifiers and/or sources). However, it is typically less expensive touse an amplifier to boost the signal at the surface, rather than to putamplifiers subsea.

The downlead fibers 122 may propagate the interrogating pulse pairs tothe sensor interferometers of the seismic sensor stations 142 (e.g.,located on the sea floor). As previously described, the use ofwavelength selective FBG reflectors in the interferometric sensorswithin the stations 142 allows for wavelength division multiplexing(WDM) of multiple stations in series on a single downlead fiber 122(e.g., N stations, with corresponding wavelengths λ1-λN).

In response to the interrogating pulse pair 310, the gratings in eachsensor station will reflect light in a corresponding wavelength channel,creating interfering pulses 320. The pulses 320 may be directed back upto the detecting portion of the instrumentation unit 110 (e.g., by acirculator 123 contained in a module connection node 144.), via uploadfibers 124. As illustrated, an array connection module 130 may beincluded to route upload and download fibers from the lead cable 120to/from appropriate series of array cable modules 140. As illustrated,the detecting components may include WDM demultiplexers 550 (e.g., onefor each upload fiber 124) that splits the different sensor wavelengthsto different detector circuits 562. Electrical signals generated at eachdetector circuit 562 may be passed to a demodulation processing unit 563to be processed, for instance by any known technique in the art toextract the sensor phases of that wavelength channel and correspondingseismic data from each sensor station 142. Demodulated sensor data fromthe processing unit be may transmitted further (via a host interface564) to a host computer for storage and quality control.

For some embodiments, a monitoring unit (not shown) after each modulatorchannel may monitor the output light signals and adjust one or moreparameters of the modulator channels accordingly. As will be describedin greater detail below with reference to FIG. 9, for some embodiments,the demodulation processing unit 563 or the host computer 570 may beconfigured to perform a layer peeling algorithm in an effort to reducethe effects of cross-talk between sensors within a station. Further, forsome embodiments, the host computer 570 may be configured to command thesource unit 510 to adjust the wavelength of the optical signalsgenerated, for example, to account for detected changes in wavelengthsof the sensor station gratings over time, for example through changes intemperature.

Combining WDM with Inline TDM and a Layer Peeling Algorithm

In order to limit the number of lead fibers it is desirable to maximizethe number of sensors that can be multiplexed on a pair of down lead andup lead fibers. As described in previous sections, this is achieved bycombining time division multiplexing (TDM) within each station withwavelength division multiplexing (WDM) between stations. For example,with 5 sensors per station (reference, 3 accelerometers and hydrophone)and N=20 wavelength channels the total number of sensors that can beinterrogated through a pair of lead fibers becomes 5×20=100. However,the fact that the sensors within a sensor are arranged inline withmultiple reflectors on the same fiber causes distortions in the detectedTDM multiplexed interference signals. The effects of these distortionscan be reduced by use of an inverse scattering algorithm, such as layerpeeling.

Multiple reflections may result in cross-talk between sensors within astation. FIG. 9 illustrates how the detected pulses comprise a compositeof reflections from all the gratings along the sensor fiber of a sensorstation. However, embodiments of the present invention may reducecross-talk interference between sensors in an OBS sensor station byapplying algorithms, for example, within the demodulation processingunit 564 located in the instrumentation unit 110 shown in FIG. 5.

For example, the host computer may be configured to apply an inversescattering algorithm to detect an accurate transmission phase delayresponse between each pair of subsequent reflectors while reducingcross-talk from other reflectors within the array. One form of inversescattering algorithm is the layer-peeling algorithm. This algorithmallows the use of gratings with higher reflectivity in a TDM systemwithout creating unacceptable cross-talk, hence improving the powerbudget and in many cases allowing a system without the use ofremote/remotely pumped amplifiers.

Signal processing software, for example, running in the hostdemodulation processing unit 564 may be used to process the lightdetection output from the detection circuitry 562 to eliminatecross-talk from higher order reflections in accordance with oneembodiment of the present invention. Suitable layer-peeling algorithmsare described in detail in the commonly owned U.S. patent applicationSer. No. 10/649,588, entitled “METHOD AND APPARATUS FOR REDUCINGCROSS-TALK INTERFERENCE IN AN INLINE FABRY-PEROT SENSOR ARRAY,” hereinincorporated by reference.

As the light pulses propagate through a sensor station they mayexperience coupling between the polarization propagation modes of thefiber. The influence of the multiple reflections on the detectedinterference signals will generally depend on this polarization modecoupling. In order to ensure accurate results from an inverse scatteringalgorithm, a polarization resolved measurements of the interferenceresponses may therefore be required. Suitable methods for polarizationresolved interrogation of the interference responses are described indetail in the previously mentioned U.S. patent application Ser. Nos.10/649,588 and 11/056,970.

Grating Array Design

As previously described, the sensors within each station 142 may beformed by a series of gratings with overlapping reflection (channel)bands. Using multiple wavelength channels, multiple stations may beinterrogated on a common sensor fiber utilizing wavelength divisionmultiplexing, as illustrated in FIG. 6A, showing three adjacentwavelength channels. As illustrated, the interrogating laser frequency(wavelength) of channel N, ν_(N) (λ_(N)), may be controlled to be withinthe grating channel bandwidth B_(ch), of grating N at all gratingoperating temperatures and all times, accounting for possible wavelengthshifts in grating spectrum over time. For some embodiments, the gratingsmay be designed for wavelength channels selected such that at a nominaltemperature (e.g., 4° C.), the wavelengths may range from approximately1530 nm (λ1) to approximately 1560 nm (λ20), with a relatively constantfrequency spacing, λν_(ch) (see FIG. 6A), between the wavelengthchannels, for example, 200 GHz.

For some embodiments, the channel bandwidth (B_(ch)) may beapproximately 25% of the channel spacing (e.g., 50 GHz assuming a 200GHz spacing). The reflectivity within the channel bandwidth may be equalor nearly equal to R (e.g., between 0.9 R and R as shown in FIG. 6A),where R can range, for example, from 1 to 10%.

The grating reflectivity of a particular grating within the otherwavelength channel bands should be kept below a level R−x, for example,with x typically >40 dB to suppress demodulation errors andinter-station cross-talk due to multiple reflections between sensorstations along the same fiber.

As alluded to above, and as shown in FIG. 6B, the reflectivity for eachgrating (R1-R6) in a sensor station may be varied to optimize thesignal-to-noise ratios for all sensors, and to reduce errors, includingcross-talk, introduced by multiple reflections between gratings. Asmentioned above, the unwanted effects caused by multiple reflections cangenerally be reduced by use of layer peeling, or other inversescattering processing techniques. However, the accuracy of the outputfrom the inverse scattering processing will generally be more accurateif the magnitudes of the errors that have to removed through suchprocessing techniques are small. Hence, for some embodiments, reductionin errors/cross-talk due to multiple reflections can be achieved byletting the reflectivities of later gratings may be stronger thanearlier gratings. For example, for some embodiments, the reflectivitiesfor the six sensors may be as follows: R₁=4.0%, R₂=4.5%, R₃=5.0%,R₄=5.5%, R₅=6.0%, and R₆=6.5%.

Further, the (optical) distance, L, of fiber between the center positionof (any) two gratings forming a sensor may for example be in the rangefrom 4 to 20 meters. At nominal operating conditions, L should equal onehalf of the distance between the two optical pulses in one interrogatingpulse pair, corresponding to the delay in the aforementionedcompensating interferometer, CIF.

Band Wavelength Division Multiplexing (B-WDM)

While the array cable module 140 shown in FIG. 2 utilizes a singlesensor fiber 146 for interrogating all sensor stations 142 (withcorresponding wavelengths λ1-λN), for some embodiments, band wavelengthdivision multiplexing (b-WDM) may be utilized to divide the wavelengthsof a array cable module onto two (or more) sensor fibers. In otherwords, each sensor fiber may carry one band of wavelengths, and eachsensor fiber may then be coupled to the sensor stations in the arraycable module having corresponding wavelengths within the correspondingwavelength band.

The use of B-WDM in a connection node may minimize the variation inreflected power levels from the stations throughout an array cablemodule. In contrast, if all sensors are along a single fiber, there maybe a substantial difference in loss seen by the first sensor in theseries and the last sensor due to the distributed loss throughout thesensor array (each sensor station has some transmission loss).

As illustrated in FIGS. 7A and 7B, for some embodiments, a moduleconnection node 744 may include a band-WDM unit 745 downstream from acirculator 723 configured to divide wavelengths in an optical signalreceived from a circulator 723 into groups of wavelengths in differentbandwidths. The groups of bandwidths may be carried on multiple sensorfibers, such as sensor fibers 746 and 747 shown in the figures. Theband-WDM may be any suitable type component or components, such as aC-band red/blue splitter or a C/L-band splitter. For some embodiments, aC-Band (˜1525-1565 nm) and L-band (˜1570-1610 nm), may be used toapproximately double the number of useable wavelengths compared to theuse of a single band (e.g., a C-band only).

For some embodiments, the series of sensor stations 742 may be evenlydistributed on the multiple fibers. For example, assuming an array cablemodule with N stations, N/2 sensor stations (e.g., 1 to N/2) may beinterrogated with sensor fiber 746 and N/2 sensor stations (e.g., N/2+1to N) may be interrogated with sensor fiber 747.

As illustrated, a multi-fiber tube, such as fiber in metal tube 748 maybypass each station eliminating the need for corresponding cutting andsplicing at each station, while only a sensor fiber need be cut tosplice into the sensor housing. As previously described, at a junctionbetween array cable modules, the circulator 723 may couple a differentfiber from the multi-fiber tube 748 to a sensor fiber leading into theB-WDM unit 745.

The multiple sensor fibers may also be housed in one or more tubes. Asillustrated in FIG. 7A, multiple sensor fibers carrying differentwavelength bands may also be housed in a single protective tube, such asa fiber in metal tube (FIMT) 749. If the FIMT 749 and both sensor fibersare cut in order to connect the appropriate sensor fiber to the sensorsof the station, a splice 751 may be needed at each station, even for thesensor fiber that is not connected to the sensors of the station. Asillustrated in FIG. 7B, however, for some embodiments, multiple FIMTsmay be provided for the multiple sensor fibers. For example, asillustrated, each sensor fiber 746 and 747 may have its own FIMT 749 and747, respectively.

OBS Station Interrogation from Two Ends

In applications, such as OBS, that involve substantial material,manufacturing and installation costs, it is often desirable to design insome degree of redundancy to allow continued interrogation of at leastsome sensors in the event of a failure. Examples of such failuresinclude, but are not limited to, breakage (or other type damage) to alead cable, damage to one or more fibers contained in a cable, orfailure or cable breakage. In any case, some embodiments of the presentinvention provide sensor topologies with inherent redundancy that allowsensor stations to be interrogated from multiple directions.

In the present description, the term direction refers to the directionthat interrogating (and reflected) pulses travel relative tointerrogated sensor stations, in different modes of operation. In otherwords, in a first (e.g., normal) mode of operation, interrogating pulsesmay travel to a sensor station from one direction, while in a second(e.g., redundancy-enabled) mode of operation, interrogating pulses maytravel to the sensor station from another direction.

FIGS. 10A and 10B illustrate an exemplary OBS sensing system topologyallowing interrogation from different directions before and after abreak 941 in a lead cable (e.g., 141 ₁) or an array cable module 140, inaccordance with one embodiment of the present invention. The redundancyprovided by the illustrated topology allows sensor stations 142 locatedboth before and after the break 941 to be interrogated. The illustratedtopology utilizes a connection 950 of cables 141 to interrogatedifferent series 960 of array cable modules 140 extending from an arrayconnection module 930.

In normal operation, all stations along series 960 ₁ and 960 ₂ (togetherthese form an array cable) are interrogated via lead cable 141 ₁. Inthis arrangement, the lead cable 120 may contain twice as many fibers asthe lead cable in a system without redundancy (e.g., that shown in FIG.1A). All fibers will normally be connected to the corresponding fibersin all the lead cables (i.e., 141 ₁, 141 ₂, etc.). To be able tointerrogate sensors at both sides of one break 941, optical power may becoupled to one extra downlead fiber 122 from the instrumentation (e.g.,by adding an extra 1×2 splitter) and one extra uplead fiber 124 may beinterrogated at the receiver end by coupling this extra fiber to anextra WDM Demultiplexor 550. The number of detector circuits does nothave to increase since the number of interrogated stations will be thesame, but the connections from the WDM Demultiplexors 550 to thedetector circuits may need to be rearranged to couple the reflectedlight from the different stations to the corresponding detectorcircuits.

As illustrated in FIG. 10A, in a first (e.g., normal) mode of operation,a first series 960 ₁ of array cable modules 140 may be interrogated viaa first lead cable 141 ₁ extending from an array connection module 930.As described above, the lead cable 141 ₁ may include multiple fibers,allowing N sensor stations 142 (e.g., with wavelengths λ1-λN) in acommon array cable module 140 to be interrogated via a common sensorfiber 146. Connection nodes 144 may be used to couple a different fiberfrom the lead cable 141 ₁ to the sensor fiber 146 of a subsequent arraycable module 140. As will be described in greater detail below, withreference to FIGS. 12 and 13, different designs of connection nodes 144may facilitate interrogation from multiple ends, with the exactcomponents utilized depending on the particular embodiment.

Thus, as illustrated in FIG. 10B, in a second mode of operation (e.g.,upon occurrence of an event, such as a break 941 in the lead cable 141 ₁or some other type of failure preventing interrogating pulses 910 orreflected interference pulses 920 from being transmitted to or fromsensor stations 142 in the same or subsequent array cable modules 140),at least some of the sensor stations 142 in the series 960 ₁ may beinterrogated via a lead cable 141 ₂, from the opposite direction. Asillustrated, the connection 950 may allow interrogating pulses 930carried in one or more fibers of a lead cable 141 ₂ to reach sensorstations 142 in the series 960 ₂ occurring after the break 941. Theconnection 950 may also allow reflected/interfering pulses 940 to becarried back to surface instrumentation via the lead cable 141 ₂. Incase of breakage in the lead cable 120, array connection module 930, orlead cables 141 (141 ₁, 141 ₂, . . . , t, etc.) these components mayhave to be replaced.

FIGS. 11A and 11B illustrate another exemplary OBS sensing systemtopology that allows interrogation from two ends, in accordance with oneembodiment of the present invention. Rather than utilize a connection950, as shown in the topology of FIGS. 10A-10B, the topology shown inFIGS. 11A and 11B utilize separate lead cables 120 ₁ and 120 ₂ andseparate (or common) array connection modules 930 ₁ and 930 ₂. The leadcables will normally have the same number of fibers as the lead cable ina system without redundancy (cf. FIG. 1A). Otherwise, interrogation ofsensor stations 142 in both directions may be carried out in arelatively similar manner. In normal operation, light will only betransmitted through the fibers in one of the lead cables (e.g., 120 ₁).

As illustrated in FIG. 11A, before the occurrence of a failure, sensorstations 142 in a first array cable 1060 ₁ of array cable modules 140may be interrogated as described above, with the first lead cable 120 ₁used to carry interrogating pulse pairs 1010 and reflected pulses 1020.After a break 1041, as shown in FIG. 11B, the second lead cable 120 ₂may be used to interrogate sensor stations in the first array cable 1060₁ in the other direction with pulse pairs 1030 and to carry reflectedpulses 1040 back to the instrumentation.

A potential advantage of the topology shown in FIGS. 11A-11B over thetopology shown in FIGS. 10A-10B is that the array cable covers a longerdistance for the same number of array cable modules, since the arraycables in FIGS. 10A-10B are folded.

As illustrated in FIGS. 11A and 11B, for some embodiments, the leadcables 120 ₁ and 120 ₂ may have separate corresponding array connectionmodules 930 ₁ and 930 ₂, respectively. For some embodiments, the arrayconnection module 930 ₂ may be installed during installation of thearray cables 1060 (1060 ₁, 1060 ₂, etc.), but without initially runningthe corresponding “redundant” lead cable 120 ₂. By installing the arrayconnection module 930 ₂ in this manner, the lead cable 120 ₂ may bedeployed only if a cable or station failure is detected, thereby atleast deferring cost, while still accommodating deployment of the leadcable 120 ₂ and, thus, enabling redundant interrogation at a later time.

Exemplary Connection Nodes

While FIGS. 10A-10B and FIGS. 11A-11B illustrate different sensorstopologies that allow for interrogation from two sides, FIGS. 12A-12Band FIGS. 13A-13B illustrate examples of different module connectionnodes that may be used with either of the sensor topologies shown inFIGS. 10A-10B and FIGS. 11A-11B.

Referring first to FIGS. 12A and 12B, array cable modules 140 utilizingmodule connection nodes 1144 with a pair of circulators 1149 and acoupler 1147 are shown. As shown in FIG. 12A, “forward” interrogationmay take place as described above, with pulse pairs 1110 used tointerrogate sensor stations 142 in a first array cable module 140carried in a first lead fiber 1150. The pulse pair 1110 is directed to asensor fiber 1146 coupled to the sensor stations 142 through the bottomcirculator 1149 and the coupler 1147. Reflected pulses 1120 are thencarried from the sensor stations 142 through the coupler 1147 and theupper circulator 1149 to a return fiber 1151. The second array cablemodule 140 is interrogated in a similar manner, through another leadfiber 1152 and return fiber 1153.

Referring to FIG. 12B, however, after a cable break 941 occurs betweenstations in the first array cable module 140, “backside” interrogationfrom the other end of the series of array cable modules 140 may beenabled. Backside interrogation may be automatically enabled, forexample, after automatically detecting a cable break 941 by theinstrumentation, as indicated by a lack of reflected pulses fromstations located after the break.

In any case, as illustrated in FIG. 12B, while sensor stations 142located before the break (e.g., λ1) are reachable in a conventionalmanner, sensor stations (e.g., λ2-λN) located after the break 941 arenot reachable in this arrangement. However, sensor stations 142 in thesubsequent array cable module 140, although located after the break 941are reachable. For example, as illustrated interrogating pulse pairs1110 on a lead fiber 1154 from the “backside” direction may be guided toa sensor fiber 1146 of the sensor stations 142 via the top coupler 1149(in the module connection node 1144 of the array cable module).Resulting reflected pulses 1120 are then carried from the sensorstations 142 through the coupler 1147 and the bottom circulator 1149 toa return fiber 1155.

An advantage of the arrangement is that, even when interrogation occursfrom the “backside” direction, as shown in FIG. 12B, the sensors withinthe station 142 are still interrogated in the normal order. For example,as shown by the reflected pulses 1120 carried on the lead fiber 1155,the reference interferometer (R) may still be interrogated first,followed by X, Y, and Z accelerometers and finally the hydrophone (H).Interrogating the reference first may be advantageous because it willnot suffer from cross-talk from the other sensors in the station. Itshould be noted, however, that some amount of optical loss occurs witheach path taken through the couplers 1147.

FIGS. 13A and 13B illustrate another exemplary configuration of arraycable modules that allows for bidirectional interrogation in accordancewith another embodiment of the present invention. In the illustratedarrangement, array cable modules 140 utilizing module connection nodes1144 with circulators 1149 only, without couplers, are shown. As shownin FIG. 13A, “forward” interrogation may take place as described above,with pulse pairs 1110 used to interrogate sensor stations 142 in a firstarray cable module 140 carried in a first lead fiber 1150 to a sensorfiber 1146 via a circulator 1149, which also directs resulting reflectedpulses 1120 to a return fiber 1151.

Referring to FIG. 13B, however, after a cable break 941 occurs betweenstations 142 in the first array cable module 140, backside interrogationfrom the other end may be enabled. As in the previous arrangement shownin FIGS. 12A and 12B, the first station (λ1) located before the break941 is reachable in the forward direction, while the remaining sensorstations (λ2-λN) in the same array cable module 140 are not. However, inthe arrangement shown in FIGS. 13A and 13B, these remaining sensorstations, although located after the break 941, may be reachable in thebackside direction.

As illustrated, for some embodiments, when interrogating in the“opposite” direction, a connection node 1144 in one array cable module140 may be used to couple lead and/or return fibers (1156 and 1157) to asensor fiber 1246, via a circulator 1149. In this manner, interrogatingpulses 1110 from the backside may be used to interrogate these sensorstations (λ2-λN) occurring after the break 941 in another array cablemodule 140 by guiding light from the backside lead fiber 1156 to asensor fiber 1246 via a circulator 1247. Reflected pulses 1120 may bedirected back onto the return fiber 1157.

This technique does have an advantage in that sensor stations 140located after a break 941 may still be interrogated. However, theaccuracy of measurements obtained from the reflected/interfering pulses1120 may be reduced somewhat (relative to measurements obtained viainterrogation in the first direction), in part due to fact that thesensor stations are being interrogated in the wrong direction. Forexample, as shown by the reflected pulses 1340, the sensors, in order,as seen during backside interrogation: hydrophone (H), Z, Y, and Xaccelerometers, and finally the reference interferometer (R).

By interrogating the reference interferometer last, the referencereading may be affected by cross-talk from the hydrophone, which mayreduce resolution. However, depending on the application, the reducedresolution may be acceptable, particularly given the ability tointerrogate sensors located after a cable break. Further, for someembodiments, a second reference interferometer (not shown) may beincluded and positioned such that it occurs earlier (first) in theoptical path when a sensor station is interrogated from the backside.

CONCLUSION

Features and aspects of any of the embodiments described herein can becombined or otherwise modified with any features or aspects of otherembodiments described herein. While the foregoing is directed toembodiments of the present invention, other and further embodiments ofthe invention may be devised without departing from the basic scopethereof, and the scope thereof is determined by the claims that follow.

1. An array of ocean bottom seismic sensor stations, comprising: atleast one array connection module; a plurality of array cables extendingfrom the array connection module, each array cable including a series ofarray cable modules and a multi-fiber lead cable, each array cablemodule including a series of seismic sensor stations suitable fordeployment at or below the ocean bottom floor and connected bymulti-fiber lead cables, each station housing a plurality of opticalsensors, all stations within the same array cable module sharing acommon sensor fiber; for each array cable module, a module connectionnode to couple a different one or more fibers of the lead cable to asensor fiber used to interrogate a series of corresponding seismicsensor stations; and wherein the sensors within each station are formedby a series of gratings with overlapping wavelength reflection bands,each corresponding to a range of wavelengths.
 2. The array of claim 1,wherein the reflectivity of later gratings is stronger than earliergratings.
 3. The array of claim 1, wherein the optical distance of fiberbetween the center position of two gratings forming a sensor is in therange of 4 to 20 meters.
 4. The array of claim 1, wherein the opticaldistance of fiber between the center position of two gratings forming asensor is equal to one half the distance between the two optical pulsesin one interrogating pulse pair.
 5. The array of claim 1, wherein thereflectivity within the wavelength range of the wavelength reflectionbands is between .9 R and R, where R ranges from 1 to 10 percent.
 6. Thearray of claim 5, wherein the grating reflectivity of a particulargrating within the wavelength range of other wavelength reflection bandsis below a level R−x, with x above approximately 40 dB, to suppressdemodulation errors and inter-station cross-talk.
 7. The array of claim1, wherein the gratings are designed for wavelength reflection bandsselected such that at a nominal temperature, the correspondingwavelength range is from approximately 1530 nm to approximately 1560 nm.8. The array of claim 1, wherein the gratings are designed with arelatively constant frequency spacing between the wavelength reflectionbands.
 9. The array of claim 1, wherein the bandwidth of a wavelengthreflection band is approximately 25 percent of the spacing between thewavelength reflection bands.
 10. A sensing system, comprising: at leastone array connection module; a plurality of array cables extending fromthe array connection module, each array cable including a series ofarray cable modules and a multi-fiber lead cable, each array cablemodule including one or more series of sensor stations connected bymulti-fiber lead cables, each station housing a plurality of opticalsensors sharing a common wavelength, the common wavelength beingdifferent for each station in a same series, all stations within thesame array cable module sharing a common sensor fiber, the sensorswithin each station being formed by a series of gratings withoverlapping wavelength reflection bands; for each array cable module, amodule connection node to couple a different one or more fibers of thelead cable to a sensor fiber used to interrogate a series ofcorresponding seismic sensor stations; and instrumentation coupled tothe series of sensor stations and configured to interrogate sensorshoused in each station using time division multiplexing and tointerrogate sensors housed in different sensor stations in each seriesusing wavelength division multiplexing.
 11. The system of claim 10,wherein the reflectivity of later gratings may be stronger than earliergratings.
 12. The system of claim 10, wherein the instrumentation isconfigured to use an inverse scattering processing technique.
 13. Thesystem of claim 12 wherein the inverse scattering technique is a layerpeeling technique.
 14. An array of seismic sensor stations, comprising:at least one array connection module; a plurality of array cablesextending from the array connection module, each array cable including aseries of array cable modules and a multi-fiber lead cable, each arraycable module including a series of seismic sensor stations connected bymulti-fiber lead cables, each station housing a plurality of opticalsensors, all stations within the same array cable module sharing acommon sensor fiber; for each array cable module, a module connectionnode to couple a different one or more of fibers of the lead cable to asensor fiber used to interrogate a series of corresponding seismicsensor stations; and the sensors within each station are formed by aseries of gratings where the reflectivity of later gratings is strongerthan earlier gratings.
 15. The array of claim 14 wherein the sensorswithin each station are formed by a series of gratings with overlappingwavelength reflection bands corresponding to a wavelength range.
 16. Thearray of claim 14, wherein the reflectivity within the wavelength rangeof the wavelength reflections bands is between .9 R and R, where Rranges from 1 to 10 percent.
 17. The array of claim 14, wherein thegrating reflectivity of a particular grating within the wavelengthranges of other wavelength reflections bands is below a level R−x, withx above approximately 40 dB, to suppress demodulation errors andinter-station cross-talk.
 18. The array of claim 14, wherein the opticaldistance of fiber between the center position of two gratings forming asensor is in the range of 4 to 20 meters.
 19. The array of claim 14,wherein the optical distance of fiber between the center position of twogratings forming a sensor is equal to one half the distance between thetwo optical pulses in one interrogating pulse pair.
 20. The array ofclaim 14, wherein the gratings are designed for wavelength reflectionbands selected such that at a nominal temperature, the wavelength rangeis from approximately 1530 nm to approximately 1560 nm.
 21. The array ofclaim 14, wherein the gratings are designed with a relatively constantfrequency spacing between the wavelength reflection bands.
 22. The arrayof claim 14, wherein the bandwidth of a wavelength reflection band isapproximately 25 percent of the spacing between the wavelengthreflection bands.